# tf.compat.v1.flags.register_validator

<br />

Adds a constraint, which will be enforced during program execution.

#### View aliases


**Compat aliases for migration**

See
[Migration guide](https://www.tensorflow.org/guide/migrate) for
more details.

[`tf.compat.v1.app.flags.register_validator`](https://www.tensorflow.org/api_docs/python/tf/compat/v1/flags/register_validator)

<br />

    tf.compat.v1.flags.register_validator(
        flag_name, checker, message='Flag validation failed',
        flag_values=_flagvalues.FLAGS
    )

The constraint is validated when flags are initially parsed, and after each
change of the corresponding flag's value.
Args:
flag_name: str, name of the flag to be checked.
checker: callable, a function to validate the flag.
input - A single positional argument: The value of the corresponding
flag (string, boolean, etc. This value will be passed to checker
by the library).
output - bool, True if validator constraint is satisfied.
If constraint is not satisfied, it should either return False or
raise flags.ValidationError(desired_error_message).
message: str, error text to be shown to the user if checker returns False.
If checker raises flags.ValidationError, message from the raised
error will be shown.
flag_values: flags.FlagValues, optional FlagValues instance to validate
against.
Raises:
AttributeError: Raised when flag_name is not registered as a valid flag
name.